Move Download to its own page again.

The front page was a bit too cluttered with it there.
Change 'open source' to 'free content'.

Freedoom has always established itself as free software, the change to
open source was sudden and made its own name seemingly contradictory.

Freedoom is not source code nor software, however, so call it 'free
content'.
Reword twenty years into "over the decades"

Avoids the page going out of date based solely on how increasingly far
in the past 1993 was.